1. Field of the Invention
This invention relates to processes for and apparatuses for use in classifying an original to be reproduced in lithography according to its density distribution. The invention also relates to processes for and apparatuses for use in adjusting a setting up a color separation tone curve for an electronic color separator for reproducing in lithography a color original in a desired tone.
2. Description of the Prior Art
When a color reproduction is to be produced from a color original including transparent and reflection copies, the original must be color separated to form typically four negatives or positives each corresponding to the respective separated colors of the original. Electronic color separators or scanners are widely used to separate the colors of the original. The scanner scans an original which may be a transparency or a reflection copy to measure the density of the original and separate the colors of the original as well as to effect desired modification of the reproduction such as masking, various color corrections, etc. by means of electronic circuitry. Although the scanner is useful in significantly reducing the length of the color separation operation and in producing reproductions of excellent quality, it entails several problems that need to be solved.
One of the problems is derived from the fact that each original has its own particular color characteristics and density characteristics. A desired color separation negative or positive cannot be obtained by simply adjusting an original on a scanner and pushing the start button. This is because the color materials of the original and the reproduction are different in color characteristics, and particularly because their density ranges differ. Typically, the color materials of color film originals and reflective originals have a wider density range and printing inks including a coloring agent have a narrower density range. Therefore, when a color separated negative is to be prepared from a color film original, the density range of the negative must be trimmed. Also, in lithography, color density is expressed by the size of the printing dots, but the density of the original is not directly proportional to the size of the screen dots. Therefore, in order to reproduce the desired tone of the original in a reproduction, the tone of highlight regions as well as the tone of shadow regions of the reproduction must be precisely controlled in addition to proper adjustment of the tone of middle tone regions.
According to the current practice, the respective densities of the highlights, shadows and halftones are individually measured by means of a densitometer, and on the basis of the measured data and the personal knowledge and skill of an experienced scanner operator, the control knobs of the scanner are adjusted so that an optimum desired reproduction is obtained by selecting the density range of the separated film and the separation tone curve. However, such adjustment of the color separation tone curve of the scanner must be achieved by a large number of control knobs on the scanner, requiring skill and time.
In order to save man hours and the cost of the color separation, an experienced, skilled color retoucher closely observes the color original and classifies it on the basis of personal experience, according to its density distribution. Then the scanner is adjusted according to this classification. Even with such adjustment of the scanner, only about forty percent of the resulting reproductions are satisfactory for use. When the reproduction is not satisfactory, the scanner must be readjusted and another separation film must be produced. Therefore it is desirable to provide a simple, objective and efficient technique for classifying an original according to density distribution. It is furthermore desirable to provide a simple and efficient technique for adjusting a scanner.